Metal treating process

ABSTRACT

THERE IS DISCLOSED A METHOD FOR APPLYING PROTECTIVE CHEMICAL COATINGS TO METAL SURFACES TO IMPROVE THEIR PAINT ADHESION PROPERTIES AND/OR CORROSION RESISTANCE BY SEQUENTIALLY SPRAYING ONTO SAID SURFACES, THROUGH A NOZZLE SYSTEM ON A SINGLE HAND WAND, A CLEANING SOLUTION AT HIGH PRESSURE SUCH AS 300-600 P.S.I., A CHEMICAL COATING SOLUTION AT A LOW PRESSURE SUCH AS 80 P.S.I., AND A FINAL PASSIVATING RINSE. WATER RINSES MAY ALSO BE APPLIED THROUGH THE SAME HAND WAND MOUNTED NOZZLE SYSTEM. THE METHOD IS CAPABLE OF APPLYING ZINC PHOSPHATE COATINGS TO FERRIFEROUS AND ZINCIFEROUS SURFACES, AS WELL AS OTHER TYPES OF CHEMICAL COATINGS TO A WIDE VARIETY OF METAL SURFACES.

United States Patent 01 fice 3,684,588 Patented Aug. 15, 1972 3,684,588METAL TREATING PROCESS John Patrick Curran, Doylestown, Pa., assignor toAmchem Products, Inc., Ambler, Pa. No Drawing. Filed May 20, 1970, Ser.No. 39,160 Int. Cl. C23f 7/08 U.S. Cl. 148-615 Z 6 Claims ABSTRACT OFTHE DISCLOSURE There is disclosed a method for applying protectivechemical coatings to metal surfaces to improve their paint adhesionproperties and/or corrosion resistance by sequentially spraying ontosaid surfaces, through a nozzle system on a single hand wand, a cleaningsolution at high pressure such as 300600 p.s.i., a chemical coatingsolution at a low pressure such as 80 p.s.i., and a final passivatingrinse. Water rinses may also be applied through the same hand wandmounted nozzle system. The method is capable of applying zinc phosphatecoatings to ferriferous and zinciferous surfaces, as well as other typesof chemical coatings to a wide variety of metal surfaces.

This invention relates to the art of applying chemical coatings to metalsurfaces to prepare them for painting or to increase their corrosionresistance. It particularly involves a novel method of applying suchcoatings. The method is capable of use with a wide variety of protectivecoatings for metals, but it is of particular utility in the field ofapplying zinc phosphate coatings to ferriferous and zinciferoussurfaces. For this reason, the following discussion will be castprincipally in the context of zinc phosphate coatings.

Various application systems have been used to apply protective coatings,including zinc phosphate coatings, to metals, and while these vary insome respects, practically all systems involve a cleaning step in whichcleaning solution is contacted with the surfaces, a coating step inwhich coating solution is contacted with the surfaces, and a final rinsestep in which a passivating rinse is applied to the coated surface. Inaddition, water rinsing steps are often included preceding or followingeach of the three principal steps.

When the metal parts which are to be coated are formed into individualarticles, the foregoing sequence of steps is usually performed in aspray tunnel having a number of separate stations for each step in theprocess. The parts to be coated are carried through the spray tunnel ona conveyor and as they pass through each processing station, one step inthe process is performed. Thus, the equipment involved includes not onlythe spray tunnel and its conveyor equipment, but also a solution tank,spray risers, and pumping equipment for each processing step, includingthe water rinsing steps. The solution tanks for each processing stageare very large, compared to the volume of solution in contact with themetal parts being processed at any given moment, and for reasons ofeconomy, the solution falling out of contact with the parts is recycledinto the solution tank even though it is partially chemically depleted.In order to maintain uniformity of coating quality it is necessary tomake control measurements of the solutions in the various solution tanksand to add make-up material to those tanks as it is needed.

Sometimes one or more of the spray coating stations in a tunnel-typesystem is replaced by a dip tank large enough to accommodate parts beingcoated as they move along the conveyor. Such systems still involve theuse of large volumes of solution and require control measurements andsolution replenishment.

From the foregoing it can be seen that the spray tunnel system forapplying protective coatings to metal parts involves a large capitalinvestment, and requires careful operational control. These factors, asa practical matter, limit its use to mass production operations wheremany parts of moderate size are to be substantially continuously givenprotective coatings. This has limited the field of use of protectivecoatings for metals and has effectively denied their advantages to manypotential users. For example, very large parts cannot be treated in atunnel system unless a commensurately large tunnel is provided, one solarge that it is economically impractical. It is also impractical to usesuch a system in the field, such as at a construction site, or at thelocation of an installed machine which is being repaired andrefurbished. The tunnel technique is difiieult to adapt to productionsituations where parts of varied size and shape must be processed andthe number of any given type part is relatively small. Finally, someplants have a need to operate a metal coating installation only on anintermittent, sporadic basic and cannot justify the large capitalinvestment in tunnel-type equipment.

Attempts have been made in the past to devise methods for applyingprotective coatings "to metal which do not have the disadvantages of thetunnel or dip tank techniques and which forego as few of the advantagesof those techniques as possible. However, such methods have not hadwide-spread success. One method which has been employed is to introducethe coating solution into the steam line of a steam cleaning apparatus.The solution is discharged with the steam through a nozzle onto thesurface. The nozzle can be mounted on the end of a hand wand which ispassed over the surface of the metal being treated. Thus, this systemcan be used in the field, or in intermittent operations, or with verylarge parts.

The steam application method has severe limitations, however, which havecircumscribed its success. For one thing, chromate coatings and zincphosphate coatings cannot be suitably applied, and the method istherefore effectively limited to the application of iron phosphate toferriferous surfaces. Since the solutions are applied at very hightemperatures, any volatile components are driven from the solution. Thusnitrite ion will be evolved as oxides of nitrogen. One furthercharacteristic of zinc phosphate coatings which makes them unsuitablefor application by the steam process is that such coatings, when fresh,are particularly susceptible to the rapid formation of light rust in ahot humid environment, such a phenomenon being termed rust blushing.When the application of a zinc phosphate coating is attempted by thesteam method, it is found that the surfaces rust blush severely.

In accordance with the present invention a new method of applyingprotective coating is provided which does not require the large capitalinvestment, and complex op erating procedures of the spray tunnel anddip tank systems, and which overcomes the disadvantages and limitationsof the steam application system. The method involves the application ofcleaning solution, coating solution, final rinsing solution, and anydesired Water rinses sequentially through a nozzle system mounted on asingle hand wand. Thus the method is adaptable to field use, to use withlarge parts, to short or intermittent production runs, and to coatingsituations where flexibility in coating time, cleaning time, and thelike is required. The method can be used with a wide variety ofprotective coating systems, and in this respect is more flexible thanthe steam application method, which, as noted, can only be used withiron phosphate coatings. In particular, the method the invention iscapable of applying good quality zinc phosphate coatings to ferriferousand zinciferous surfaces.

In accordance with the method of the invention the metal to be coated isfirst cleaned by a cleaning solution passed through the nozzle of thehand Wand at high pressures in the range of 300-600 p.s.i., preferablynear 500 p.s.i. Then the surface may be rinsed, if desired, and coatingsolution is sprayed onto the surface at a relatively low pressure,preferably about 80 p.s.i., through the same hand wand mounted nozzlesystem. Following application of the coating chemicals the surface mayagain be rinsed, if desired, and then a final passivating rinse solutionis sprayed onto the surface through the nozzle of the hand wand, at arelatively low pressure, preferably about 80 p.s.i.

The application pressures which have been outlined generally above havebeen found to be quite important to the successful operation of themethod. Performance of the cleaning step at a high pressure is ofparticular importance since such pressures are necessary to obtainquick, thorough and uniform cleaning of the metal. When cleaning isperformed at lower pressures, not only is more time required for thecleaning step, but the likelihood of producing non-uniform low qualitycoatings is substantially increased.

The use of relatively low pressures during application of the coatingmaterial itself is also of considerable importance in the application ofthe method inasmuch as the use of high pressures, above about 80 p.s.i.,has been found to result in the formation of sparse, non-uniformcoatings, low coating weights, and at best, greatly increasedapplication times to obtain a useable coating.

The application pressure for the final passivating rinse is not ascritical as that for the application and cleaning steps, but sincecompletely satisfactory results are obtained at low pressures, it ispreferred for reasons of convenience to use such low pressures.

The temperatures of application are also important factors in the methodof the invention. It is preferred that the cleaning step be performed ata relatively low temperature between about room temperature and 150 F.both for reasons of economy and to minimize the contribution of thecleaning step to rust blushing when zinc phosphating is being done. Itis preferred that the coating materials be applied at relatively lowtemperature also, for example between 110 F. to 150 F. The use ofcoating temperatures toward the high end of this range results in morerapid coating formation, although uniform coatings can be obtainedthroughout the temperature range. If coating temperatures above 150 F.are used in the coating step of the method, the likelihood of rustblushing is increased, as is the likelihood of evolution of volatilecomponents such as the loss of nitrite ion as oxides of nitrogen. Thetemperatures of the passivating rinse step and of water rinses arepreferably about 150 F. In field use the method can thus be readilypracticed at a single temperature which is suitable for all of theprocessing steps including the water rinse steps. The considerationsoutlined above are illustrated by the following examples.

EXAMPLE 1 The effect of the application pressure for the cleaning stepof the method is illustrated by the following tests. A mild titanatedalkaline cleaner was sprayed for 30 seconds onto a series of lightlyoiled steel panels at varying pressures for 30 seconds. The applicationtemperature was 110 F. After water rinsing, a conventional zincphosphate solution was then applied to the panels for one minute at 70p.s.i., and the coating appearance was rated visually. Table 1 reportsthe results of this series of tests. In Table 1 the column headed waterbreak represents an evaluation of the cleanliness of the work asmeasured by the uniformity with which the water left the surface withoutbeading up.

From the foregoing it can be seen that high cleaning pressures arerequired to obtain satisfactory coatings.

EXAMPLE 2 pressure, p.s.i.

Coating appearance No coating. Sparse coating.

hin, uniform coating. Uniggrm coating.

Table 2 shows that in the low pressure ranges of about 20 to aboutp.s.i. good phosphate coatings are produced. It should also be notedthat at the application temperature used in this test, no rust blushingwas encountered.

EXAMPLE 3 A further series of tests was made to illustrate the effect ofcoating solution application temperature upon coating quality. In thisseries of tests the coating application pressure was maintained at 80p.s.i., while the coating solution temperature was varied. Aconventional zinc phosphate coating solution was employed, and thecoating time was one minute. The cleaning step was performer at 500p.s.i. and at 150 F. The results are reported in Table 3.

TABLE 3 Coating temperature, F. Coating appearance Rust blush 70 Thinuniform coating None. Good uniform coating. Do. do.. Do. .do Slight. doSevere.

The foregoing tests show that good coatings can be ob tained over arelatively broad range of application temperatures without encounteringrust blushing.

In accordance with the method of the invention the large solution tankscharacteristic of the spray tunnel and dip tank systems are eliminated.Cleaning material, coating material, and passivating rinse material areprovided in small tanks or drum of concentrated pre-mix solution and aremetered into water supplied from the water mains as it is pumped to thenozzle system on the hand wand. The pre-mix tanks or drums need not beheated, when the water from the mains is hot or is heated by anauxiliary heater. Mixing valves or similar devices associated with thespray Wand feed equipment may be utilized to adjust the proportion ofeach of the pre-mixes fed to the water from the mains in order todeliver to the nozzle system solutions of the appropriate dilution.

As was pointed out above, the method may be used with the various kindsof coating solutions known to the art. Generally speaking, theproportions of components of any particular type of coating solution atuse dilution are preferably approximately the same as those used inspray tunnel systems. For particular coating situations, it may bedesirable to alter the relative proportions or concentrations somewhatwithin the general ranges taught by the art. When zinc phosphate coatingsolutions are used, and when nitrite ion is used as an accelerator, inaccordance with the art, care should be taken to maintain the phosphatesolution in the pre-mix tank at a sufficiently low concentration thatthe nitrite ion is not dissipated as evolved nitrogen dioxide gas. Whentitanated cleaner, such as a mild alkaline cleaner containing col- Iloidal titanium salts, is used for the cleaning step, the cleanerpre-mix solution should be stirred to maintain the colloidal salt ingood suspension.

Treating times for each of the steps of the method, including waterrinsing steps, may be varied over a considerable range, in accordancewith the judgment of the operator as he observes the results of hiswork. Generally speaking, adequate final coatings can be obtained bycontacting each section of the surface to be coated with cleaningsolution for about 30 seconds, with the coating solution for about 30seconds, and with the final rinse for about 30 seconds. Similarly, thewater rinses, if employed, may be applied for about 30 seconds.

Since the method of the invention involves the sequential spraying ofsolutions through the Wand at widely differing pressures, it may bedesirable to employ a nozzle having alternately useable orifices ofdifferent sizes, or to employ a multiplicity of nozzles on the wandwhich are alternately operated. For this reason, the terms nozzle andnozzle system are employed herein in a broad sense to connote not only asingle nozzle, but also such specially refined nozzle systems.

I claim:

1. A method for applying a protective chemical coating to a metalsurface comprising sequentially delivering to said surface through anozzle mounted on a hand held wand a cleaning solution at a pressurebetween about 300 p.s.i. and about 600 p.s.i. and a temperature betweenabout 70 F. and about 150 F., a chemical coating solution at a pressurebetween about 20 p.s.i. and about 80 p.s.i. and a temperature betweenabout 110 F. and about 150 F., and a final passivating rinse solution.

2. A method in accordance with claim 1 in which a portion of the aqueouscomponent of each of said solutions is supplied from a main, in whichthe remaining components and the remainder of the aqueous component aresupplied as concentrated premixes, and in which each of said solutionsis formed by admixture of its premix and said aqueous component portionin the course of delivery thereof to said hand wand mounted nozzle.

3. A method for applying a zinc phosphate coating to ferriferous andzinciferous surfaces comprising sequentially deliverying to the surfaceto be coated through a nozzle mounted on a hand held wand an alkalinecleaning solution at a pressure between about 300 p.s.i. and about 600psi. and a temperature between about F. and about 150 F., a zincphosphate coating solution at a pressure between about 20 psi. and aboutpsi. and a temperature between about F. and about F., and a finalpassivating rinse solution.

4. A method according to claim 3 and further comprising delivering waterrinses to said surface between delivery of said cleaning and saidcoating solutions, and between delivery of said coating and said finalrinse solutions.

5. A method according to claim 4 in which said solutions and rinses aredelivered to said surface at substantially the same temperature.

6. A method according to claim 3 wherein said hand held wand is providedwith a multiplicity of nozzles which are alternately operated fordelivering said solutions in the sequential order set forth.

References Cited UNITED STATES PATENTS 2,228,836 1/1941 MacQuaid 117104X 3,207,165 9/1965 Durrant 134-36 X 3,167,797 2/1965 Hergonson 134-99 X3,615,912 10/1971 Dittel et al. 148--6.15 R

RALPH S. KENDALL, Primary Examiner WESTON, Assistant Examiner US. Cl.X.R.

1l7--104 B, 104 R; 134--36

